In this work we investigate a low dimensional semiconductor system, in which the light-matter interaction is enhanced by the cooperative behavior of a large number of dipolar oscillators, at different frequencies, mutually phase locked by Coulomb interaction. We experimentally and theoretically demonstrate that, owing to this phenomenon, the optical response of a semiconductor quantum well with several occupied subbands is a single sharp resonance, associated to the excitation of a bright multisubband plasmon. This effect illustrates how the whole oscillator strength of a two-dimensional system can be concentrated into a single resonance independently from the shape of the confining potential. When this cooperative excitation is tuned in resonance with a cavity mode, their coupling strength can be increased monotonically with the electronic density, allowing the achievement of the ultra-strong coupling regime up to room temperature.
@article{arxiv.1212.4422,
title = {Charge induced coherence between intersubband plasmons in a quantum structure},
author = {Aymeric Delteil and Angela Vasanelli and Yanko Todorov and Cheryl Feuillet-Palma and Margaux Renaudat St-Jean and Grégoire Beaudoin and Isabelle Sagnes and Carlo Sirtori},
journal= {arXiv preprint arXiv:1212.4422},
year = {2012}
}